The present invention relates to a power transmission system for a four-wheel drive motor vehicle, and more particularly to a system having fluid-operated multiple-disk friction clutches for distributing torque of an engine to front and rear wheels of the vehicle and for controlling the torque to the right and left driving wheels wherein the torque distribution ratio to the front and rear wheels and to the right and left wheels are controlled in accordance with driving conditions of the motor vehicle and road surface conditions.
Japanese Patent Application Laid-Open 61-155027 discloses a system in which a pair of multiple-disk transfer clutches for front wheels and for rear wheels are provided in a transfer device for controlling the distribution of the torque to the front wheels and the rear wheels.
Japanese Patent Application Laid-Open No. 62-103227 discloses a system for controlling the differential operation of a differential disposed between right and left wheels. In the system, a fluid operated multiple-disk friction clutch is provided between a differential case and a side gear of a differential for restricting the differential operation. Actuating pressure of liquid for the clutch is controlled for changing the differential operation restricting torque so as to improve the driveability and maneuverability of the motor vehicle.
In order to improve the driveability and maneuverability of the vehicle, it is preferable to provide both the torque distribution device and the differential restricting device in a four wheel drive vehicle. However, a hydraulic control circuit having control valves for controlling the clutches must be provided in pairs and oil pumps must be provided so that there is a problem of arranging these elements in the power transmission system. Namely, the clutches and the control systems are preferably disposed in a case housing the transmission or the differential.
On the other hand, the multiple-disk friction clutch uses the oil (automatic transmission fluid) in the automatic transmission as actuating oil for actuating and lubricating the clutch, since the oil has a good friction characteristic and low oil viscosity. On the other hand, a final reduction gear of the differential provided on the outside of the differential case is composed of a hypoid gear. In such a differential, the extreme pressure agent which contains additives improves the ability to adhere to the surfaces of metals under high bearing pressures. Such an agent is liable to cause the multiple disks of the clutch to stick and to slip, which cause unpleasant vibrations of and noise from the vehicle body.
Moreover, the clutch comprises a clutch drum and a clutch hub, each having a plurality of disks, and a piston oil chamber formed between the clutch drum and a piston. The piston is slidably mounted on the clutch drum with splines for pushing the disks of the clutch drum to the disks of the hub. When the clutch drum rotates, the piston rotates together with the clutch drum. When the clutch rotates at a high speed, the centrifugal force of oil in the piston oil chamber is exerted on the piston. When disengaging the clutch, the pressure of the oil is reduced. However, the centrifugal force acts to push the piston to the disks of the clutch drum, so that the clutch cannot be completely disengaged. Further, the centrifugal force causes various disadvantages in accurate control of the pressure of oil.